Likely fixes issue #7 (untested).

Without this field, U2F just isn't working and the accounts with U2F
set up are shown an OTP prompt.

There seems to be no good reason to change group membership when
exchanging tokens, let's try just passing the old one over.

Using CORS-enabled requests in the background.

Allow loading remote images.

Simplifies the device manager code as we don't have to duplicate the
IP/network matching logic there.

Should make the subdivision between apps (idp, sso) more obvious.